1. Field of the Invention
The present invention generally relates to an optical scanning device for scanning a light beam emitted from a light emitting section onto a target scanning surface. More particularly, the present invention relates to an optical scanning device capable of controlling a light amount to control a feedback light to a light emitting section of a light source of the optical scanning device.
2. Description of the Related Art
In conventional optical scanning devices, a polygon mirror or a galvanic mirror has been generally used as a deflector for scanning a light beam. On the other hand, there has been a growing demand for forming high-resolution images and fast printing. To that end, it is necessary to increase the rotation speed of the mirror. However, there is a limit of fast scanning by a method of rotating the mirror due to the durability of the bearing and heat and noise generated by windage loss.
To overcome the problem, research of a deflection device fabricated using silicon micromachining technology has been continually carried out. One method has been proposed in which a vibration mirror and a torsion beam axially supporting the vibration mirror are integrally formed on a Si substrate. The vibration mirror formed on the Si substrate may be called a MEMS (vibration) mirror, where MEMS stands for Micro Electro Mechanical Systems and refers to a device integrated on a Si substrate and the like.
According to the deflection method of the vibration mirror, the size of the mirror surface can be reduced and accordingly, the size of the vibration mirror can also be reduced; and the mirror is moved back and forth based at a resonance frequency and therefore, fast deflection can be achieved with lower noise and less consumption power. Further, the vibration becomes lower and little heat is generated, and therefore, the housing including the optical scanning device may become thinner. Further, even when low-cost resin forming material having a low blending ratio of glass fibers is used, the image quality is hardly degraded.
Patent Documents 1 and 2 disclose examples where the vibration mirror is used instead of the polygon mirror. However, the vibration mirror described in Patent Documents 1 and 2 may have problems that the resonance frequency may vary due to the change of the spring constant of the torsion beam supporting the vibration mirror and that the deflection angle of the vibration mirror may also vary due to the change of the viscosity resistance of air caused by the change of air pressure.
To overcome the problems, a technique is proposed, as disclosed in Patent Document 3, of stabilizing the deflection angle by detecting the deflection angle by detecting a scanned light beam in advance, and controlling a current applied to the vibration mirror.
Further, an optical scanning device using the vibration mirror, or an image forming apparatus is disclosed in, for example, Patent Documents 4 and 5.
According to the inventions described in Patent Documents 3 through 5, by using the vibration mirror instead of the polygon mirror, it becomes possible to reduce noise and energy consumption. Further, by using the vibration mirror as the optical deflector of the image forming apparatus, it becomes possible to provide an image forming apparatus suitable for an office environment. Further, the housing of the optical scanning device can be thinner due to lower vibration, and as a result, the cost and weight can also be reduced.
Patent Documents 6 and 9 are also prior art documents related to the present invention, though not all of the documents describe the vibration mirror used as the optical deflector. Patent documents 6 and 7 disclose a light beam characteristics measurement method and a device capable of estimating the depth of the characteristics required for the light beam.
Patent Document 8 discloses a technique in which a density unevenness of an image is reduced by controlling the APC light amount of plural light sources, i.e., a light amount, to be constant in a non image forming period but excluding a feedback light affecting period. Herein, the non image forming period refers to a period other than an image forming period.
Patent Document 9 discloses an invention in which, by not adjusting an amount of light of the light source at a timing when the incident angle of the light beam to the reflection surface of the polygon mirror which serves as an optical deflector is substantially 90 degrees, an initializing process of a photo detector (hereinafter referred to as “PD”) can be stably carried out, the PD being incorporated in a light source section including a laser diode (hereinafter referred to as “LD”) as a light source.    Patent Document 1: Japanese Patent No. 2924200    Patent Document 2: Japanese Patent No. 3011144    Patent Document 3: Japanese Patent No. 3445691    Patent Document 4: Japanese Patent No. 3543473    Patent Document 5: Japanese Patent Application Publication No. 2004-279947    Patent Document 6: Japanese Patent Application Publication No. 2000-9589    Patent Document 7: Japanese Patent No. 3594813    Patent Document 8: Japanese Patent Application Publication No. 2006-198881    Patent Document 9: Japanese Patent Application Publication No. 2007-148356
In an optical scanning device, when the maximum deflection angle on the reflection surface of deflection means is greater than the incident angle of a light beam from light source means, a so-called “feedback light” phenomenon is observed at a certain vibration timing of the mirror (deflection means), the feedback light being a reflection light of a light beam emitted from a light source and reflected on the mirror. This feedback light may cause the increase of noise, thereby impeding stable oscillation and light emission of a laser diode.
In a case where a MEMS vibration mirror is used as the deflection means, a light beam emitted from a light source may be returned (fed back) to the light emitting section of the light source after being reflected on the reflection surface of the reflection means when the vibration mirror is arranged to be moved in a wider range than when the angle between the direction of the light beam from the light source and the direction of the reflection surface of the vibration mirror becomes substantially 90 degrees.
Further, when a light beam emitted from the light emitting section is fed back to another light emitting section, the feedback light may affect the performance of the other light emitting section. Further, when the laser diode(s) is continuously turned ON to be used for detecting the synchronization purpose during other than an image forming period, the temperature of the laser diode(s) may be increased; the light emission efficiency may be reduced; and energy consumption of the laser diode may be increased. Further, unlike polygon mirrors, the MEMS vibration mirror moves back and forth (i.e., the MEMS vibration mirror does not rotate). Accordingly, the light beam is mechanically scanned in both directions along the main scanning direction on an image surface. It is not preferable to apply sinusoidal vibration to the MEMS vibration mirror, because the scanning speed of the light beam near the maximum amplitude is remarkably reduced.
The image forming period is required to be provided while the scanning speed of the light beam is linearly changed as much as possible. In that sense, the image forming period is provided in the middle part between both the maximum amplitudes. The light beam emitted from the laser diode and reflected by the MEMS vibration mirror in a part corresponding to an area other than an image forming area (hereinafter may be referred to as a non-image forming area) may become a so-called ghost light, and a part of which may become the feedback light fed back to the light emitting section of the laser diode, which may cause the power fluctuation. Further, a part of the ghost light which reaches an image carrier such as the photosensitive body may cause to create a ghost image on the image forming surface.
When the maximum deflection angle of the vibration mirror is greater than the maximum incident angle required to scan in the image forming area, namely when the vibration mirror is arranged to move to deflect the light beam beyond the image forming area, it may become possible to prevent the feedback light from reaching the light emitting section of the laser diode by forcibly turning OFF the light beam from the light source when the deflection angle of the vibration mirror is in a range corresponding to the non-image forming area but excluding in a range for detecting the synchronization purpose. More specifically, for example, the LD (laser diode) of the light source is forcibly turned OFF after the light beam passes the PD (photo detector) for the synchronization detection, the PD being installed in the scanning range of the light beam and continued to be turned OFF while the light beam reaches the maximum amplitude and until after the light beam passes the PD again. By turning OFF the LD in the non-image forming area like this, it may become possible to reduce the unnecessary lighting of the LD and better control the temperature increase of the LD and devices near the LD, thereby achieving highly effective light emission and stable lighting of the LD. When a laser diode array (LDA) is used as the light source, it may become possible to reduce the energy consumption and achieve high-power light emission.
When plural light emission points in the light source are provided like the above LDA, a technique may be used in which an amount of light emission is controlled by adjusting a drive current, voltage, pulse width, and the like applied to each of the light emission points so that each of the plural light beams has a desired amount of light emission by performing a light amount control (a.k.a “APC” (Automatic Power Control)) at a timing when the feedback light from each light emission point may otherwise interfere with the stable light emission. Further, in a case where it is difficult to provide such a light emission amount control period as the LD(s) is forcibly turning OFF to perform the APC, the APC may be arranged not to perform the APC while the feedback light is desirably to be turned OFF, or another type of the light emission amount control period may by provided in which a driving current to drive the light emitting section is reduced to a level less than a predetermined level such a case as the amount of light emission is of the LD(s) is reduced to a level less than the threshold level for the detection by the PD. When plural light emission points are provided in the light source, it is necessary to appropriately allocate the timings for the APC among the light emission and the allocation of the light emission amount control periods when each of the light beams is forcibly turned OFF. By providing the light emission amount control period when the light beam is forcibly turned OFF in the non-image forming area, it may become possible to prevent the temperature increase caused by continuous lighting of the LD, maintain stable lighting condition, and reduce the energy consumption.
Even when the LD is unable to be forcibly turned OFF, by appropriately setting the amount of a light beam in accordance with the sensitivity of the LD when the light beam scans on a device for detecting synchronization, it may become possible to provide the light emission amount control period while, for example, the driving current applied to the LD of the light source is reduced to a level equal to or less than a predetermined level, thereby enabling performing an appropriate APC. As described above, by reducing the amount of light emission as much as possible, it may become possible to better control the occurrence of the feedback light phenomenon that a beam light emitted from a light emission point of the light source returns to a light emission point of the same light source, so that stable LD light emission may be maintained.
Based on the ratio and the phase of CW turn ON time for detecting synchronization to PD detection time, it may become possible to set the start and stop counting values determining the light emission amount control period when the light beam is appropriately and forcibly turned OFF. Further, by resetting a pixel counter when the light beam passes the PD and successively monitoring and controlling the amplitude condition of the light beam, it may become possible to appropriately set the light emission amount control period when the light beam is appropriately and forcibly turned OFF and a turn-ON period for one dot for light beam detection means. Further, by employing two-point synchronization, it may become possible to appropriately designate a writing start position in response to the operating condition of the vibration mirror influenced by disturbance. Further, by controlling the positions and the intervals of the pixels, it may become possible to form a high-quality image having less displacement.